1. Field of the Invention
The present invention relates to a liquid crystal display device and a process for producing it. More particularly, the invention relates to the connection between a connection terminal portion of a transparent electrode and an electrode connection portion formed on a drive circuit substrate in a liquid crystal display device having upper and lower substrates of flexible film.
2. Description of the Related Art
A conventional liquid crystal display device is formed such that a pair of panel substrates are laminated with a small gap between them and that a liquid crystal is filled in the gap. In such a liquid crystal display device, an anisotropic conductive film is used for connection of wiring terminals on a drive circuit substrate with transparent electrode terminals formed on a panel substrate. The anisotropic conductive film is a thermoplastic adhesive, a thermosetting adhesive, or an adhesive mass in which conductive particles are dispersed.
If an inspection finds a defect, the connection between the panel substrate and the drive circuit substrate is separated in order to allow the reuse of the substrates. There is a method known to enable the separation of the connection between the panel substrate and the drive circuit substrate, which uses two adhesives different in thermal hardening temperature for connection between the substrates. In this method, the substrates are connected at a first thermal hardening temperature. If an inspection finds a defect in a liquid crystal display unit with the substrate. thus connected, the connection is softened at a second temperature higher than the first thermal hardening temperature, to separate the drive circuit substrate from the panel substrate.
This connection method may be applied to a liquid crystal display device using a glass plate for panel substrate, but cannot be applied to those using a flexible film such as polymer film for panel substrate. A flexible film substrate reveals a low heat resisting temperature. For example, a polyester film has a glass transition point of 80.degree. C. A substrate of polyester film would have a thermal expansion and contraction at a temperature over the glass transition point of polyester, for example about 130.degree. C., which results in breakage of transparent electrodes (ITO: Indium Tin Oxide electrodes) on the substrate. If substrates are connected by the above-mentioned connection method using two adhesives different in thermal hardening temperature, a temperature difference of 20.degree.-30.degree. C. is necessary in thermal hardening temperature between the two adhesives. A thermoplastic resin such as polyester film has a softening point of about 130.degree. C., while a thermosetting adhesive normally has a thermal hardening temperature of about 170.degree. C. Therefore, the above connection method is not suitable for application to a separable connection of panel substrate.
Further, if a polyester is used as a panel substrate, a bonding strength between transparent electrodes (ITO) and the polyester substrate is lower than that between the transparent electrodes (ITO) and an anisotropic conductive film. Thus the transparent electrodes (ITO) would be stripped from the polyester substrate when the drive circuit substrate is separated from the polyester substrate for reuse. This causes difficulties in separation and in reuse of the substrates.
As explained above, conductive particles are dispersed for example in a hotmelt adhesive for connection. The conductive particles may be metal particles of nickel, solder, silver, or other metal, or, gold-plated plastic beads with a diameter of several .mu.m to several ten's .mu.m. If a panel substrate is made of glass, the conductive particles could be crushed upon heat press for connection between the panel substrate and the drive circuit substrate, thereby face-contacting with the transparent electrodes on the glass substrate. In contrast, if a panel substrate is made of polymer film, the conductive particles may damage a transparent electrode. For example, a conductive particle may break a transparent electrode and another particle may cause cracks in the transparent electrode, because the polymer film substrate is softer than the conductive particles. Such damages cause unstable contact between the polymer film substrate and the conductive particles due to a difference in thermal expansion and contraction between the materials after aging. This results in poor contact between the materials.